Chronograph screen



Jan. 30, 1940. J, v. DUNHAM CHRONOGRAPH SCREEN Filed June 28, 1959 Patented Jan. 30, 1940 UNITED STATES PATENT OFFICE 2 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) The invention described herein may be manufactured and used by or for the Governmentl for governmental purposes, Without the payment to me of any royalty thereon.

l This invention relates to a screen for use with a projectile velocity measuring chronograph or other ballistic apparatus and more particularly to a screen of the type that breaks a circuit upon passage of a projectile therethrough to effect operation of a chronograph through interruption of the circuit.

Screens of various types have been Widely used Vheretofore in the art for the purpose of detecting the passage of -a projectile past a given point by causing the projectile to pass through a screen located at such point and so arranging the screen that passage of the projectile therethrough would either make or break an electrical circuit including some form of ballistic apparatus. Of these 20 screens the wire screen is probably the best known type and generally consists of a screen frame having a wire strung back and forth in zig-zag 'fashion so as to provide a circuit covering an area that may reasonably be interposed in the trajectory of a. projectile without undue nicety in positioning the screen. In this type of screen the -projectile must break the wire to interrupt the circuit including the wire in order to actuate the ballistic apparatus, but experience has taught the dillculty in obtaining accurate velocity records of very small projectiles, and other projectiles with long points, which may readily pass between the wire strung back and forth across the screen without breaking it, moreover, where screens of this type are set along the trajectory at known spaced distances apart it frequently occurs that a projectile contacting a wire of a preceding screen will be so deflected from its normal trajectory as to entirely miss subsequent screens. To avoid the above mentioned inherent disadvantage of the wire screen in determining the velocities of small arms Weapons the first screen in the trajectory of the projectile has often been replaced by a single fine conducting wire stretched across the trajectory of the projectile at a point closely adjacent the muzzle, but obviously such an element has the disadvantage of presenting a small target and unless the ammunition is extremely accurate, it is impossible to break the wire with certainty at distances greater than approximately 25 feet from the weapon, however, distances within this limit are not always beyond the zone of continued linear acceleration of the bullet after leaving the muzzle and accordingly introduce errors in the determination of the muzzle velocity.

Another type chronograph screen in quite extensive use is the foil screen which in general comprises two parallel pieces of metal foil electrically insulated from each other and mounted in a screen frame to be disposed with the surfaces of the foil pieces in the path of the trajectory normal thereto so that when a projectile passes through them it will make momentary Contact by closing the circuit through the foil pieces and allowing the current to be used in actuating ballistic apparatus. This type of screen is adapted to be incorporated in a design aording a relatively large target and does not seriously alter the trajectory, whereby it is an easy matter to ensure a. projected missile passing through a succession of the screens; however, the screen is useless for operating certain of the standard projectile velocity measuring chronographs such as the Le Bouleng chronograph illustrated in U. S. Patent No. 1,357,281.

In View of the disadvantages of the above mentioned types of screens it is an object of this invention to provide a screen combining the advantage of the iol screen in offering a relatively large target with the advantage of the wire screen for interrupting an electric circuit, without being Vcharacterized by the disadvantage of either the foil or wire screens.

It is a further object of this invention to provide a chronograph screen presenting a relatively large target, which will not retard the night of a projectileor deflect it from its normal trajectory so that ballistic measurements of a projectile in flight based on its use will not be in .error due to retardation of the projectile by the screen.

The specific nature of the invention as well as l other objects and advantages thereof will clearly y appear from a description of a preferred embodiment as shown in the accompanying drawing in which:

Fig. 1 is an elevation of a chronograph screen embodying the invention;

Fig. 2 is a modified form of chronograph screen embodying the invention;

Fig. 3 is a section taken on line 3-3 of either Fig. 1 or 2;

Fig. 4 is a diagrammatic view of a chronograph system embodying the chronograph screen of this invention.

This application contains matter which has been derived from application Serial No. 184,252 filed January 10, 1938 and is a continuation in part thereof.

Referring now to the drawing by characters of reference and more particularly to Fig. l there is shown an open screen frame l0 which may be of rectangular or other suitable shape, having its sides recessed as indicated at II to provide for mounting therein a thin sheet screen I2 of any brittle substance that will completely shatter upon impact by a projectile. The screen I2 is retained in position within ther frame through the overlapping engagement of retaining members I3 disposed on opposed portions of the frame IIJ and securedthereto by any conventional means such as the bolts and wing nuts as indica-ted. The screen I2 must be thin enough and of such degree of brittleness as to completely shatter without measurably retarding or delecting a projectile in ight. f

For use with small arms projectiles I have found thin glass of strained or brittle character up to about 0.01 inch thick to satisfactorily function as a screen without materially retarding or delecting the projectile. Obviously, however, screens of the same character and of greater thickness may be utilized in the measurement oi the velocitiesof heavier projectiles with the same result.

An electric conductor such as a thin sheet of foil, chemically deposited silver, coat of graphite or electrically conducting powdered material is suitably secured to or deposited on the screen I2 in such manner as to be so broken or disintegrated upon shatter Iof the screen that a circuit including the conductor will be opened.

The conductor may be applied to a selected strip of the screen extending between opposed portions of the sides of the frame II) as indicated at I4 in Fig. l or it may substantially cover one side of the screen as shown in Fig. 2 at I5. In either form of the screen the retaining members I3 are made of electrically conducting material and are so arranged as to conductively engage the conductor on the screen I2. The screen is placed in circuit with any desired ballistic apparatus by proper electrical connection with the retaining members I3 which are provided with terminals I3 for the purpose.

A chronograph system is diagrammatically shown in Fig. 4 embodying the use of screens of this invention with a Le Bouleng type chronograph, In Fig. 4 a first screen SI is shown located at a known distance from the muzzle of a gun G and is placed in an independent circuit I 6 for controlling release of a recording chronorneter rod for free fall. At a known distance from the first screen a second screen S2 is disposed in the path of the trajectory of a projectile projected from gun G and is placed in a second independent circuit I1 for controlling a registrar which is adapted to fall and release a trigger that normally holds a knife in cocked position so related to the chronometer rod that it will make an indent on a recorder carried by the chronometer rod when released.

In operation the chronograph or other ballistic instrument is conditioned for recording the elapsed time between successive ruptures of the two independent circuits I6 and I'I as is well understood in the art. The screens SI and S2 are disposed a known distance from the muzzle of` gun G' along the trajectory T of a projectile, io which a velocity determination is to be made. Because of the relative large size of the targets presented by the screens and their characteristic function of being adapted to completely shatter upon impact at any point with the projectile, to ensure disintegration of the conductors I4 carried thereby andA hence positive successive rupture of the circuits I6 and II upon passage of the projectile through the screens SI and S2, the latter need not be as accurately disposed relative to the trajectory as has been imperative heretofore in the art. After the screens are4 positioned and the chronograph conditioned for recording the circuits IB and I'I are energized and the projectile projected from the gun G to successively interrupt the circuits, as is well known in the art, to actuate the ballistic instrument. It is some times desirable to include the screens SI and S2 in a plurality of circuits for operating several similar ballistic instruments simultaneously through shattering of the two screens whereby the accuracy of the velocity determination or other data being determined may be increased by taking the mean of the results recorded by the several instruments and for this purpose the screens of this invention are admirably adapted.

Having now described my invention what I claim and desire to secure by Letters Patentis:

1. A chronograph screen for detecting the passage of` a projectile embodying a screen member presenting a relatively large target comprised of glass so strained as to be entirely shatterable on impact of the projectile at any point, the thickness of the screen member being so related to the projectile to avoid signincantretardation and deflection of the projectile, and a disintegrable'l electric conductor carried by the screen member.

2. A chronograph screen comprising an open frame recessed to receive a screen member, a glass screen member 4strained so as to be completely shattered by impact with a projectile at any point positioned within the recessed part of the frame, an electric conductor disintegrable upon shattering 4of the screen member secured thereto, and electric conducting retaining members secured to said frame and overlapping the screen' member in electric conducting relation with said conductor for retaining the screen member in said frame. I

JAMES V. DUNI-IAME. 

